1. Field of the Invention
The present invention relates to a method of manufacturing an external cavity semiconductor laser, an external cavity semiconductor laser, and a wavelength multiplex transmission system.
2. Related Background Art
A semiconductor laser generates light having a desired wavelength. Such a semiconductor laser is used as a light source for optical communications. In optical communications, WDM communication is implemented using a plurality of semiconductor lasers for generating optical signals having respective wavelength components.
An example of semiconductor lasers is an external cavity semiconductor laser. The external cavity semiconductor laser has a grating fiber and a semiconductor optical amplification element. In order to obtain a stable optical output, the external cavity semiconductor laser comprises a thermoelectronic element for temperature control and a control circuit for controlling the thermoelectronic element.
If an external cavity semiconductor laser does not include a Peltier element for controlling the temperature of the semiconductor optical amplification element, the length of the optical path, i.e., the optical cavity length, is changed due to variations of ambient temperatures or injected currents. This change causes a phenomenon, i.e., mode hopping, wherein the longitudinal mode discontinuously changes. Consequently, a so-called kink appears in the I-L characteristic (current vs. optical output characteristic). At the kink, the optical output of the semiconductor laser discontinuously changes, so that the use of such a semiconductor laser degrades the transmission quality.
Therefore, it is an object of the present invention to provide a method of manufacturing an external cavity semiconductor laser capable of reducing the occurrence of kinks, an external cavity semiconductor laser, and a wavelength multiplex transmission system.
An aspect of the present invention is a method of manufacturing an external cavity semiconductor laser. This method comprises the steps of: (a) providing an optical waveguide device and a semiconductor optical amplification element along a predetermined axis, the optical waveguide device having an optical waveguide and a Bragg grating exhibiting a maximum reflectivity at a frequency fFG; and (b) arranging the optical waveguide and the semiconductor optical amplification element while monitoring a frequency fLD and intensity of light from an optical system including the semiconductor optical amplification element and the optical waveguide device.
In this method, the step of providing the optical waveguide and the semiconductor optical amplification element can include a step of adjusting a distance along the predetermined axis between the optical waveguide device and the semiconductor optical amplification element such that the oscillation frequency fLD satisfies:
0 less than fFGxe2x88x92fLD less than 20 GHz.
Another aspect of the present invention is a method of manufacturing an external cavity semiconductor laser. This method comprises the steps of: (a) preparing an optical waveguide device and a semiconductor optical amplification element, the optical waveguide device having an optical waveguide and a Bragg grating exhibiting a maximum reflectivity at a wavelength xcexFG; and (b) arranging the optical waveguide and the semiconductor optical amplification element while monitoring a wavelength xcexLD and intensity of light from an optical system including the semiconductor optical amplification element and the optical waveguide device.
In this method, the arrangement is carried out such that the wavelength xcexLD satisfies:
0 less than xcexLDxe2x88x92xcexFG less than 0.16 nanometers.
Still another aspect of the present invention is a method of manufacturing an external cavity semiconductor laser. The external cavity semiconductor laser comprises a grating fiber and a semiconductor amplification element. This method comprises the steps of (a) preparing a semiconductor optical amplification element and a grating fiber, the semiconductor optical amplification element being mounted on a stem, the grating fiber having a maximum reflectivity at a frequency fFG; and (b) arranging the semiconductor optical amplification element and the grating fiber while energizing the semiconductor optical amplification element through the stem and monitoring a frequency fLD and intensity of light from an optical system including the semiconductor optical amplification element and the grating fiber.
In this method, the arrangement can be carried out such that the oscillation frequency fLD satisfies:
0 less than fFGxe2x88x92fLD less than 20 GHz.
Still another aspect of the present invention is a method of manufacturing an external cavity semiconductor laser. The external cavity semiconductor laser comprises a grating fiber and a semiconductor amplification element. This method comprises the steps of (a) preparing a semiconductor optical amplification element mounted on a stem and a grating fiber having a maximum reflectivity at a wavelength xcexFG; and (b) arranging the semiconductor optical amplification element and the grating fiber while energizing the semiconductor optical amplification element through the stem and monitoring a wavelength xcexLD and intensity of light from an optical system including the semiconductor optical amplification element and the grating fiber.
In this method, the arrangement is carried out such that the wavelength xcexLD satisfies:
0 less than xcexLDxe2x88x92xcexFG less than 0.16 nanometers.
Still another aspect of the present invention is an external cavity semiconductor laser. The external cavity semiconductor laser has a temperature-uncontrolled-type structure and is capable of generating light having an oscillation frequency fLD. The external cavity semiconductor laser comprises an optical waveguide device and semiconductor optical amplification element. The optical waveguide device has a Bragg grating and an optical waveguide, the Bragg grating having a reflection spectrum with a maximum reflectivity at a frequency fFG, and the Bragg grating being optically coupled to the optical waveguide. The semiconductor optical amplification element is optically coupled to the optical waveguide such that the oscillation frequency fLD satisfies:
0 less than fFGxe2x88x92fLD less than 20 GHz.
An external cavity semiconductor laser of the present invention is capable of generating light having an oscillation frequency fLD. The external cavity semiconductor laser comprises a semiconductor substrate, a semiconductor optical amplification element, an optical waveguide device, and a Bragg grating. The semiconductor optical amplification element is provided on the semiconductor substrate. The optical waveguide device is provided on the semiconductor substrate and is optically coupled to the semiconductor optical amplification element. The Bragg grating is provided on the semiconductor substrate and is optically coupled to the optical waveguide. The Bragg grating has a spectrum exhibiting a maximum reflectivity at a frequency fFG. The semiconductor optical amplification element and the Bragg grating are arranged such that the oscillation frequency fLD satisfies:
0 less than fFGxe2x88x92fLD less than 20 GHz.
An external cavity semiconductor laser of the present invention is capable of generating light having an oscillation frequency fLD. The external cavity semiconductor laser comprises a substrate, an optical waveguide, a Bragg grating, and a semiconductor optical amplification element. The substrate includes at least one of LiNbO3 and LiTaO3. The optical waveguide is provided on the semiconductor substrate. The Bragg grating is provided on the semiconductor substrate and optically coupled to the optical waveguide. The Bragg grating has a spectrum exhibiting a maximum reflectivity at a frequency fFG. The semiconductor optical amplification element is optically coupled to the optical waveguide. The semiconductor optical amplification element and the Bragg grating are arranged such that the oscillation frequency fLD satisfies:
0 less than fFGxe2x88x92fLD less than 20 GHz.
An external cavity semiconductor laser of the present invention is capable of generating light having an oscillation frequency fLD. The external cavity semiconductor laser comprises an optical waveguide device and a semiconductor optical amplification element. The optical waveguide device has a Bragg grating and an optical waveguide, the Bragg grating having a spectrum that exhibits a maximum reflectivity at a frequency fFG, and an optical waveguide being optically coupled to the Bragg grating. The semiconductor optical amplification element is optically coupled to the optical waveguide. The optical cavity of this external cavity semiconductor laser is composed of the optical waveguide device and the semiconductor optical amplification element such that the oscillation frequency fLD satisfies:
0 less than fFGxe2x88x92fLD less than 20 GHz.
An external cavity semiconductor laser of the present invention comprises a semiconductor optical amplification element, a stem, a grating fiber, and a spacer member. The semiconductor optical amplification element is mounted on the stem. The grating fiber has a Bragg grating exhibiting a reflection spectrum with a maximum reflectivity at a frequency fFG. The spacer member separates the grating fiber from the semiconductor optical amplification element and defines a distance between the semiconductor optical amplification element and the grating fiber such that the oscillation frequency fLD satisfies:
0 less than fFGxe2x88x92fLD less than 20 GHz.
An external cavity semiconductor laser of the present invention is capable of generating light having an oscillation frequency fLD. The external cavity semiconductor laser comprises an optical waveguide device and a semiconductor optical amplification element. The optical waveguide device has a Bragg grating and an optical waveguide, the Bragg grating having a frequency fFG and exhibits the maximum reflectivity thereat, and the optical waveguide being optically coupled to the Bragg grating. The semiconductor optical amplification element is optically coupled to the optical waveguide device. The optical waveguide device and the semiconductor optical amplification element are arranged such that the oscillation frequency fLD satisfies:
0 less than fFGxe2x88x92fLD less than 20 GHz.
Still another aspect of the present invention is a wavelength multiplex transmission system. The wavelength multiplex transmission system comprises a first external cavity semiconductor laser, a second external cavity semiconductor laser, and an optical transmission line. The optical transmission line has an end optically coupled to the first and the second external cavity semiconductor lasers. An oscillation frequency fLD1 in the first external cavity semiconductor laser is different from an oscillation frequency fLD2 in the second external cavity semiconductor laser.